The present invention relates to a method and system for creating three-dimensional displays or images from a multiplicity of two-dimensional projected images and, more specifically, to a method and system for producing task-dependent radiographic images of an object of interest which are substantially free of blurring artifacts.
A variety of three-dimensional imaging modalities has been developed for medical applications, as well as for use in non-destructive testing of manufactured parts. In particular, a wide range of tomosynthetic imaging techniques has previously been demonstrated to be useful in examining three-dimensional objects by means of radiation. These imaging techniques differ in the size and configuration of the effective imaging aperture. At one extreme, the imaging aperture approaches zero (i.e., a pinhole) and the resulting display is characterized by images produced from a single transmission radiograph. This yields an infinitely wide depth of field and therefore no depth information can be extracted from the image. At the other extreme, the aperture approaches a surrounding ring delimiting an infinite numerical aperture resulting in projection angles orthogonal to the long axis of the irradiated object. This yields an infinitely narrow depth of field and hence no information about adjacent slices through the object can be ascertained. It therefore follows that a xe2x80x9cmiddle groundxe2x80x9d approach, which provides the ability to adapt a sampling aperture to a particular task, would be highly advantageous.
The key to achieving the full potential of diagnostic flexibility lies in the fact that perceptually meaningful three-dimensional reconstructions can be produced from optical systems having any number of different aperture functions. That fact can be exploited since any aperture can be approximated by summation of a finite number of appropriately distributed point apertures. The key is to map all incrementally obtained projective data into a single three-dimensional matrix. To accomplish this goal, one needs to ascertain all positional degrees of freedom existing between the object of interest, the source of radiation, and the detector.
In the past, the relative positions of the object, the source, and the detector have been determined by fixing the position of the object relative to the detector while the source of radiation is moved along a predetermined path, i.e. a path of known or fixed geometry. Projective images of the object are then recorded at known positions of the source of radiation. In this way, the relative positions of the source of radiation, the object of interest, and the detector can be determined for each recorded image.
A method and system which enables the source of radiation to be decoupled from the object of interest and the detector has been described in U.S. Pat. No. 5,359,637, that issued on Oct. 25, 1994, which is incorporated herein by reference. This is accomplished by fixing the position of the object of interest relative to the detector and providing a fiducial reference which is in a fixed position relative to the coupled detector and object. The position of the image of the fiducial reference in the recorded image then can be used to determine the position of the source of radiation. In addition, a technique for solving the most general application wherein the radiation source, the object of interest, and the detector are independently positioned for each projection has been described by us in co-pending U.S. patent application Ser. No. 09/034,922, filed on Mar. 5, 1998, which is also incorporated herein by reference.
Once the relative positions of the radiation source, the object, and the detector are determined, each incrementally obtained projective image is mapped into a single three-dimensional matrix. The mapping is performed by laterally shifting and summing the projective images to yield tomographic images at a selected slice position through the object of interest. A three-dimensional representation of the object can be obtained by repeating the mapping process for a series of slice positions through the object. However, the quality and independence of the tomographic images is compromised by blurring artifacts produced from unregistered details located outside the plane of reconstruction.
In addition, quantitative information has traditionally been difficult to determine from conventional tomography. Although many questions of medical interest are concerned with temporal changes of a structure (e.g., changes in the size and shape of a tumor over time), the ability to compare diagnostic measurements made over time is complicated by the fact that factors other than the parameter of diagnostic interest often contribute to the measured differences. For example, spatial variations produced from arbitrary changes in the observational vantage point(s) of the radiation source create differences between the measurements which are unrelated to temporal changes of the object being investigated. In addition, conventional X-ray sources produce radiation that varies with changes in tube potential, beam filtration, beam orientation, tube current, distance form the focal spot, and exposure time. The fluctuations in the output of radiation sources is therefore another factor that limits the ability to derive quantitative information from conventional tomography.
In light of the foregoing, it would be highly beneficial to provide a method for producing a three-dimensional representation of an object that is substantially free of blurring artifacts from unregistered details. In addition, the method should enable quantitative information related to temporal changes associated with the object to be measured.
The present invention relates to a system and a method for synthesizing an image slice through a selected object from a plurality of projected radiographic images of the selected object. The system comprises a radiation source for irradiating the object. The preferred radiation source depends upon the particular application. For example, the present invention may be practiced using x-rays, electron microscopy, ultrasound, visible light, infrared light, ultraviolet light, microwaves, or virtual radiation simulated by manipulation of magnetic fields (magnetic resonance imaging (MRI)). In one embodiment of the present invention, the position of the radiation source within a plane parallel to an image plane is determined from projected images of two object points associated with a fiducial reference which is maintained in fixed position relative to the selected object. Once the projected images are compensated for differences in magnification, the relative position of the radiation source within the plane parallel to the image plane is determined from an estimate of the actual distance between the two object points obtained from a sinusoidal fit of the distances between the projected images of the object points.
A recording medium or radiation detector is used to record a series of projected images of the selected object. The recording medium may be in the form of a photographic plate or a radiation-sensitive, solid-state image detector such as a charge-coupled device (CCD), or any other system capable of producing two-dimensional projections or images suitable for digitization or other analysis.
An image synthesizer is provided for transforming the series of projected images of the selected object into an image slice. The image slice consists of an array of pixels with each pixel having an associated attenuation value and corresponds to a cross-sectional slice through the selected object at a selected slice position. A three-dimensional representation of the object can be obtained by repeating the transformation at a series of slice positions through the object.
In addition, an optional source comparator is provided for adjusting the radiation source to enable meaningful quantitative comparisons between projected images recorded either at different times and/or using different radiation sources. The source comparator is positionable between the radiation source and the radiographic medium for producing a gradient image indicative of characteristics associated with the output from the radiation source. In operation, the source comparator is used to record a first gradient image using a first radiation source at the same time that a first projected image or series of projected images is recorded. When a second projected image or series of projected images are to be recorded, the source comparator is used to record a second gradient image. The second gradient image is compared to the first gradient and differences between the two gradient images are noted. The beam energy, filtration, and beam exposure associated with the radiation source used to record the second gradient image are then adjusted to minimize the differences between the first gradient image and the second gradient image.
In one embodiment, the source comparator comprises two wedges or five-sided polyhedrons of equal dimension having a rectangular base and two right-triangular faces. The triangular faces lie in parallel planes at opposite edges of the base such that the triangular faces are oriented as mirror images of each other. As a result, each wedge has a tapered edge and provides a uniformly increasing thickness from the tapered edge in a direction parallel to the plane of the base and perpendicular to the tapered edge. The wedges are arranged with the base of one wedge adjacent to the base of the other wedge such that the tapered edges of the two wedges are at adjacent edges of the base. One wedge is formed from a uniform high attenuation material while the other wedge is formed from a uniform low attenuation material. Accordingly, when the source comparator is irradiated from a radiation source directed perpendicularly to the bases of the wedges, the resulting image will be a quadrilateral having an intensity gradient that is maximized in a particular direction.
In operation, the system of the present invention is used to produce an image slice through the selected object that is substantially free of blurring artifacts from unregistered details located outside a plane of reconstruction. The radiation source and recording medium are used to record a series of two-dimensional projected images of the selected object. The series of two-dimensional projected images are then shifted by an amount and in a direction required to superimpose the object images of the two-dimensional images. The shifted two-dimensional images can then be combined in a non-linear manner to generate a tomosynthetic slice through the selected object. In one embodiment, the two-dimensional images are combined by selecting details from a single projection demonstrating the most relative attenuation at each pixel. Alternatively, a different non-linear operator could be used wherein the two-dimensional images are combined by selecting details from a single projection demonstrating the least relative attenuation at each pixel in the reconstructed image. Optionally, a series of reconstructed images at varying slice positions through the selected object are determined to create a three-dimensional representation of the selected object.
Alternatively, the system of the present invention is used to synthesize a three-dimensional reconstruction of the object from as few as two projected images of the object. A first projected image of the object is recorded in a first projection plane and a second projected image is recorded in a second projection plane. Each of the first and the second projected images are then rendered at a common magnification. Using a known angle between the first and the second projection planes, the first and the second projected images are transformed to occupy the same volume. The transformed first and second projected images are then combined into a three-dimensional representation of the selected object. Additional projected images are optionally combined with the three-dimensional representation to refine the three-dimensional representation.
In yet another embodiment, the system of the present invention is used to synthesize a three-dimensional representation of the selected object from two or more sets of projected images of the selected object. The first and second sets of projected images are tomosynthetically transformed into a series of contiguous slices forming a first and a second three-dimensional volume, respectively, using previously disclosed methods (e.g., U.S. Pat. No. 5,668,844) or those in the public domain (e.g., tomosynthesis). The first and second three-dimensional volumes are then rendered at a common magnification. The second three-dimensional volume is then rotated by an angle corresponding to the angular disparity between the first and the second three-dimensional volumes. The rotated second three-dimensional volume is then merged with the first three-dimensional volume to produce a three-dimensional representation of the selected object.
Alternatively, the system of the present invention can be used to determine temporal changes in the selected object. The radiation source and recording medium are used to record a first series of two-dimensional projected images of the selected object. At some later time, the radiation source and recording medium are used to record a second series of two-dimensional projected images of the selected object. Both series are tomosynthetically converted into a series of slices via previously disclosed methods (TACT(copyright)) or those in the public domain (tomosynthesis). Each slice of the first series is then correlated with a corresponding slice of the second series to form pairs of correlated slices. Each pair of slices is then aligned to maximize the overlap between homologous structures. Each pair of correlated slices is then subtracted to produce a difference image. Each difference image is then displayed individually. Alternatively, all of the difference images can be overlapped to yield a complete difference image corresponding to the volumetric difference associated with the entire tomosynthetically reconstructed volume.
When a three-dimensional representation of the selected object is produced, the three-dimensional representation can be viewed holographically using a display in accordance with the present invention. The display comprises stereoscopic spectacles which are worn by an observer and a target operatively associated with the spectacles. Accordingly, as the observer changes his or her vantage point, movement of the spectacles translates into a corresponding movement of the target. A detector is operatively associated with the target for tracking movement of the target. The detector is connected to a monitor such that the monitor receives a signal from the detector indicative of movement of the target. In response to the signal from the detector, the monitor displays an image pair of the three-dimensional representation which, when viewed through the spectacles produces a stereoscopic effect. The image pair which is displayed is changed to compensate for changes in the vantage point of the observer.